Electrostatographic latent images are typically developed using a magnetic brush development station. A magnetic brush development station typically includes a magnet with a mass of iron oxide particles or ferromagnetic powder magnetically attracted to the magnet in a chain-like arrangement. The chain-like arrangement of the magnetic particles simulates a brush. A powder toner, of pigmented or non-pigmented particles for example, is dispersed within the arrangement of magnetic particles and mixed so as to be triboelectrically charged so that the developer material includes toner powder and magnetic carrier particles of opposite polarity. The charged particles of toner are then used for the development of a latent image charge pattern formed on an image support member. The general concept of magnetic brush development has been disclosed in U.S. Pat. Nos. 2,786,439 and 2,786,440 and 2,768,441. Common magnetic brush development stations include magnetic roller(s) which carry the developer material, including the ferromagnetic carrier particles and powdered toner, into operative association with the latent image charge pattern on an image support member. When resulting rotating magnetic brush fibers move relative to the latent image charge pattern, the contact of the magnetic brush fibers with the image support member enables some of the charged toner particles to transfer to the charge pattern of the latent image.
Although such magnetic brush development stations are widely used in electrostatographic reproduction apparatus (such as copiers, printers, or the like), there are some inherent problems associated therewith. In most instances, when unoxidized, or partially oxidized iron oxide, stainless steel, magnetite or soft ferrite are used as the carrier particles, some such particles are also carried across the development gap into contact with the image support member by the arrangement of magnetic brushes. In such magnetic brushes, the magnetic core is stationary while a rotating shell provides for the transportation of the developer material across the surface of the image support member. Accordingly, the conductivity of one of these ferromagnetic particles provides the effect of a development electrode with very close spacing to the latent image. When the carrier particles become less conductive with age, for example due to toner scumming, the developing electric field is lowered which in turn reduces the amount of toner transferred to the image. In order to address this short coming, the use of small carrier particles has been promoted. The typical size of the carrier particles that are utilized range form 50 to 200 microns and these particles are coated with a polymer to optimize toner charging, prevent toner scumming and loss of carrier conductivity. For such carrier particle sizes, the amount of toner concentration usually ranges from 1 to 6 percent by weight of the developer material.
Another major concern with magnetic brush development stations of this type is the removal of counter charge. As the toner particles are attracted by the electrostatic latent image, the opposite charge of the carrier particles is exposed. This counter charge lowers the effective electrical field available for development and eventually leads to a reduction in toner development. In addition, most types of the carrier particles used in such development modes are prone to mechanical brittle failure of the particle surface. This surface fracture or breaking up of the carrier surfaces not only changes the triboelectric behavior of the developer material, but also changes the developer conductivity and makes the material more sensitive to changes in relative humidity. Various approaches have been suggested for maintaining sufficient electrical conductivity of the carrier particles by techniques such as incorporating conductive fillers or additives to the carrier coatings (see for example U.S. Pat. Nos. 4,822,708; 5,330,874; 5,332,638 and 5,272,037, the disclosures of which are incorporated herein by reference). Various methods are also practiced which help with the relative humidity stability of such carriers (see U.S. Pat. No. 4,920,023, incorporated herein by reference).
The use of insulative carrier in the development of latent image charge patterns eliminates the need for maintaining the electrical conductivity. Such use of insulative carrier is particularly useful for developing a fringe-field image. Carrier used for such applications comprise ferromagnetic material such as iron, nickel or other soft ferrite whose surface is uniformly coated with an insulative resin. However, insulative developers have an extremely difficult time getting rid of the counter charge left on the insulating carrier surface which can lead to shutdown of the entire development even at slow speeds.
In the case of soft ferrite or iron oxide powered carrier, the developer shell is rotated around a fixed magnetic core. The developer material is carried across the development zone in the vicinity of the image support member. The developer chains with such non-permanent or soft magnetic carrier particles are essentially carried across the development zone without any rearrangement or alteration. The spent developer then falls off the surface of the magnetic brush shell once it has traveled sufficiently away form the stationary magnets in the development roller. Once in the developer station sump, the spent developer can lose its counter charge.
More recently, hard magnetic materials have been used to carry and deliver the toner particles to the electrostatic image on the image support member. Many of the problems encountered with "soft" type particles are solved with the use of "hard" ferrite as carriers. However, because the magnetic attraction between the permanent magnetic core and the permanently magnetic hard ferrite carrier is so high, the developer station has to be significantly modified. One such developer station is described in U.S. Pat. No. 4,707,107, the disclosure of which is incorporated herein by reference. In the case of soft ferrite and iron oxide powder carriers, the developer station shell is rotated around a fixed magnet. When developers based on hard ferrite carrier particles are used, the magnetic core of the development roller (which contains between 4 to 30 magnetic material elements arranged sequentially in north-south pole alignment) is rotated. This causes the chains of the magnetic carrier particles, which form the development brush, to flip end-to-end at very high rates. As such, it is well recognized that the hard magnetic material carriers are not analogous to the soft carrier materials.
U.S. Pat. Nos. 4,546,060 to Miskinis et al. and 4,473,029 to Fritz et al. teach the use of hard magnetic materials as carrier particles and an apparatus for the development of electrostatic images utilizing such hard magnetic carrier particles, respectively. These patents require that the carrier particles comprise a hard magnetic material, meaning a magnetic material exhibiting a coercivity of at least 300 Oersteds when magnetically saturated and exhibiting an induced magnetic moment of 20 EMU/g when in an applied magnetic field of 1000 Oersteds. The terms "hard" and "soft" when referring to magnetic materials have the generally accepted meaning as indicated on page 18 of Introduction to Magnetic Materials by B. D. Cullity published by Addison-Wesley Publishing Company, 1972.
The biggest advantage of using hard ferrite carrier particles in a magnetic brush development station is that there are large number of chain flips taking place as the magnetic core is being rotated while the shell is essentially stationary. The number of chain flips can range form between 500 to 25,000 flips per minute. The consequence of these carrier chain flips is that there is no build up of the counter charge in the development zone. Any counter charge resulting in this development technique is bled away once the carrier chain flips and contact the magnetic brush shell. The size of the hard ferrite carrier particle employed in this method typically range from 20 to 40 microns. By using such small carrier particles, it is possible to maintain a toner concentration of between 5 and 15 percent by weight. With such small carrier particles, the development gap is generally between 0.4 and 1 millimeters and the carrier core resistance is of the order of 11 to 14 Log ohm-cm.
Although both of the described methods for magnetic brush development are practiced widely, there are certain limitations with each of them. For example, since both these methods utilize a cylindrical magnetic brush, the size of the development nap is determined by the height of the developer on the magnetic brush and the diameter of the roller. Very frequently, with such magnetic brush, the development time available in the development nap for a latent image is not sufficient to enable the complete development of the image. To address this problem, a plurality of magnetic brush rollers may be arranged in series to act sequentially on the same latent image to completely develop such image. It is not uncommon to have four development rollers arranged in serial fashion when 100 to 150 micron iron oxide or soft ferrite carrier particles are being used. As mentioned above, the counter charge left on the carrier surface of this type can stop the development of the toner to the image support member. This behavior is also responsible for making it necessary to use more than one magnetic developer brush.
On the other hand, when hard carrier particles are being used, the counter charge is constantly being removed by the large number of carrier chain flips taking place on the development roller surface. Therefore, the problem related to counter charge build-up which slows down the toner development, does not exist. However, the high magnetic strength of the rotating magnetic core does not permit the use of a series of magnetic brushes with respective rotating magnetic cores. This is because of the extremely high amounts of heat which would be produced by the Eddy currents generated by the action of adjacent magnetic cores rotating in close proximity. As a result of this difficulty, it is not possible to arrange a series of developer rollers to increase development efficiency. Further, as a result of the development station configuration and the magnetic properties of the carrier used, the hard ferrite carrier does not perform well in a station arranged for using iron oxide, magnetite, stainless steel or soft ferrite carrier particles, and vice versa.